The Present Application relates, generally, to a heat sink, a cooling module and a coolable electronic board that cools semiconductor integrated circuits, LED devices, power devices and other heat-generating bodies, and, more particularly, to a heat sink, a cooling module and a coolable electronic board that, by effectively decreasing the temperature gradient in the base without losing durability or reliability, reduces temperature increases in the heat-generating body (or namely, increases the efficiency of cooling of the heat-generating body).
Many semiconductor integrated circuits, LED devices, power devices and various other devices and electronic components that generate large amounts of heat are used in electronic equipment, industrial equipment and automobiles and the like. This is because the current flowing through the interior of these devices and electronic components generates heat in these devices and electronic components. However, a problem exists in that once the heat generated by these devices raises their temperature to above a certain level, their proper operation cannot be guaranteed, and the heat may affect other components, resulting in the possibility of deterioration in the performance of the electronic equipment or industrial equipment. In order to cool such heat-generating bodies, heat sinks having a plurality of fins have conventionally been used, and so heat sinks of various shapes and having various modifications have been proposed.
Generally, it is understood that the heat sink absorbs the heat radiated from the heat-generating body and is radiated into the air from the fins, thus cooling the heat-generating body. Various heat sinks, such as those disclosed in the following documents, have been proposed based on this viewpoint (the contents of each of the following references are incorporated in their entireties herein:                Japanese Patent Application No. 2006-237366 discloses a heat sink wherein the lowest portion crosses a plane perpendicular to a side plate that contains the axis of rotation of an axial fan (see, for example, Claim 4, as well as paragraphs 0014, 0019, 0023, 0025, etc.). Alternatively, the '366 Application also discloses a heat sink wherein the heights of the respective fins become higher the further they are away from the portion that crosses a plane perpendicular to the side plate that contains the axis of rotation of an axial fan (see, for example, paragraphs 0021, 0027, 0032, etc.).        Japanese Patent Application No. 2005-251892 discloses a heat sink that increases the cooling effect on the whole by concentrated cooling of heat-radiating fins provided near the center in contact with the heat-generating body. That is, it discloses a heat sink that utilizes the fact that the heat-generating body is easily mounted in the central area of the rear surface of the base, so when heat is conducted from the base to the heat-radiating fins, this increases the heat distribution ratio of the corresponding heat-radiating fins near the central area.        Japanese Patent Application No. H8-195453 discloses a heat-radiating plate upon which fins are disposed at stipulated intervals in a radial manner from the center of the base. In addition, the '453 Application discloses a heat-radiating plate wherein there are height differences between each fin.        Japanese Patent No. 2,744,566 discloses a heat radiator wherein the base constituting the heat sink is formed in the shape of a concave surface that becomes lower toward the center. Due to the concave surface, heat-radiating fins protruding from the base are taller near the center of the base, and thus a differential in the pressure loss arises compared to the fins on the periphery. Due to this differential in the pressure loss, the cooling air generated from the cooling fan is concentrated in the central area of the heat sink, so the central area where large amounts of heat are generated is effectively cooled.        Japanese Patent Application No. JP 2003-086984 discloses a heat sink with slits provided on the base. In particular, it discloses a heat sink wherein a first slit is provided so as to follow along a heat-receiving surface upon a heat-transfer plate protruding on the side opposite the heat-receiving surface, and a second slit is provided upon the side surface of the heat-transfer plate in a direction perpendicular to the heat-receiving surface. In addition, J the '984 Application also discloses a heat sink whereby the cooling effect is increased by varying the depths of the first slit and the second slit.        Finally, Japanese Patent Application No. 2005-277193 discloses a heat sink wherein a core portion is disposed along the centerline of an axial fan, and thin plate-shaped fins are disposed in radial directions about the core portion.        
However, each of these references places emphasis upon increasing the cooling efficiency by efficiently transferring the heat of the heat-generating body from the base of the heat sink to the fins. In particular, the heat-generating body is often in contact with the central area of the rear surface of the heat sink. For this reason, most of the conventional technology is established from the technical viewpoint of cooling the center of this central area, and thus increasing the cooling effect upon the whole.
When addressing the cooling of a heat-generating body, focus generally centers on the decrease of the temperature generated by heating the heat-generating body. However, the problem of heating a heat-generating body is not one of measuring the degree of the decrease in temperature, but rather of measuring whether the problem of the temperature of the heat-generating body exceeding a reference value occurs as a result of the decrease. Thus, rather than focusing on the temperature generated by the heat-generating body, the focus should be on the temperature of the heat-generating body using room temperature as a reference. In other words, rather than focusing on how much the heat-generating body can be cooled, the focus should be on the performance of the heat sink from a cumulative standpoint, or to what degree the temperature increase of the heat-generating body can be suppressed.
Referring to FIG. 20, which illustrates a schematic diagram illustrating the change in temperature of a heat sink and a heat-generating body, the heat sink is provided with a base 201 and fins 202. The heat-generating body 200 is in contact with the central area of the rear surface (heat-receiving surface) of the base 201. The temperature of the heat-generating body 200 is calculated as follows using room temperature as a reference. As shown in FIG. 20, the outside edges of the fins 202 are at the same temperature as room temperature. The heat from the heat-generating body 200 is conducted to the base 201. The heat from the base 201 is conducted to the fins 202. And the heat from the fins 202 is radiated to the outside environment.
Thus, considering the temperature of the heat-generating body 200 on the basis of room temperature, the temperature gradient that arises between the outside environment and the fins 202 (because heat is radiated from the heat-generating body 200, this means that it is possible to measure the temperature of the heat-generating body 200 even by considering the temperature difference between the outside environment and the bases of the fins 202) is defined to be ΔT1. Similarly, the temperature gradient from the bases of the fins 202 to the heat-receiving surface of the base 201 is ΔT2. Moreover, the temperature gradient from the heat-receiving surface of the base 201 to the heat-generating body 200 is ΔT3. Accordingly, the temperature of the heat-radiating body when room temperature is used as the reference is given by ΔT1+ΔT2+ΔT3. If this temperature of the heat-radiating body, as determined by the sum of ΔT1−ΔT3, exceeds a stipulated value, then this causes malfunctions in the heat-generating body.
The references cited above are thought to have emphasized either Policy One—increase the rate of conduction by which heat from the heat-generating body is conducted to the fins, or Policy Two—increase the efficiency of cooling in the vicinity of the central area of the base with which the heat-generating body is in contact.
However, if Policy One is emphasized, conduction occurs readily solely in the direction of thickness of the base, so a problem occurs wherein the effect of reducing the heat flux in the direction of thickness of the base is weakened. If this effect of reducing the heat flux in the direction of thickness of the base is weakened, there is a problem in that the effect of reducing the temperature gradient in the base is also weakened. This is because the temperature gradient in the base is determined by the product of the thermal resistance of the base and the thermal flux which indicates the ease of conduction of heat. With the references, in order to lower this thermal resistance, Reference 2 focused on the shape of the base, while References 1 and 3 focused on the height of the fins. As a result, while the thermal resistance in the direction of the thickness of the base becomes smaller, the thermal flux becomes larger as the thermal resistance becomes smaller, and as a result the temperature gradient becomes larger.
On the other hand, Policy Two is expressed as a technique for concentrating cooling in the vicinity of the central area, as in Reference 4. In this case, there is a focus upon heat transport only in the direction of the thickness of the base, so the thermal resistance in the thickness direction becomes small. However, the thermal flux becomes large and as a result the temperature gradient becomes larger. As is clear from FIG. 20, in order to suppress increases in the temperature of the heat-generating body, it is necessary to consider both the base and the fins; however, there is thought to be little consideration given to the temperature gradient in the base. Moreover, in Reference 4, the problem of durability, by making the base concave, arises.